Heat recovery systems are provided in a system to take full advantage of the total energy being produced by an engine. A large amount of energy produced in an engine is lost through the exhaust system and jacket water cooling. Many of the known heat recovery systems use various forms of heat exchangers in the exhaust system to convert the heat in the exhaust to a form that subsequently performs useful work. The steam-based Rankine bottoming cycle heat recovery principle is well known and has the potential to increase total engine performance by utilizing the engine exhaust to perform useful work. One process frequently used converts water to steam and uses the steam to operate miscellaneous services, such as heaters, and to drive a steam turbine. One of the major problems encounter when using the engine exhaust system to convert water to steam is soot fouling of the heat exchanger or boiler that is located in the exhaust system. This soot fouling problem is even more pronounced on systems used in diesel engines. It has been found that the soot thickness is strongly dependent on the temperature of the walls of the tubes in the boiler that are located in the exhaust system. Naturally the more tubes that are used coupled with the lower temperature used in the tubes, the more collection of soot. To optimize a system to accept a 5% loss due to sooting could add 30% to the size and cost of a total system.
Another major problem that is associated with the low tube temperature in the boilers that are located in the exhaust system is the tendency for various gases to precipitate out of the exhaust gas due to the lower tube temperature. These gases form oxides that chemically attack the various metals in the exhaust system thus shortening their useful life.
Various forms of turbine have been used in the Rankine bottoming cycle systems. These range from single or multiple stage high pressure tubines to single or multiple stage low pressure turbines and to a mix of low pressure and high pressure turbines. The main objectives considered in determining the type of turbine to use is maximizing performance, controlling the cost versus performance, and controlling total package size. It has been generally found that the more stages used in the turbine, the greater the system efficiency. However, both the cost and size of the turbine increases with an increase in the number of stages used. Many times the cost of added stages will increase at a much higher rate than that of the system performance.
Of the dual pressure turbines used, some of them direct the high pressure steam to only the high pressure side and the low pressure steam to the low pressure side and subsequently to the outlet port. These normally fail to efficiently use all of the available work in the steam or they have to use many turbine stages thus adding significantly to the total cost. Of the other dual pressure turbines used, the exhaust steam from the high pressure stage or stages is directed to a mixing chamber where it mixes with a low pressure steam and directed into the low pressure stages. The mixing chamber is primarily provided to ensure that the temperature, pressure, and velocity of the steam from the high pressure stage is respectively equalized with the temperature, pressure, and velocity of the low pressure steam priior to entering the low pressure stages. The mixing chamber that is normally used inreases the size of the total package and further adds the possibility of unwanted turbulence in the chamber.
The present invention is directed to overcoming one or more of the problems as set forth above.